how do i know i need a reboot after a console based "upgrade"?

[Tomoki Taniguchi]

when updating a system with the newest packages using the desktop
tools like synaptic,
if a package is upgraded that requires a reboot to take effect,
there is a little icon that shows up on the upper panel to let me know
i need to reboot.


on a server system,  if i do a "sudo apt-get upgrade" or "sudo aptitude upgrade"
is there anyway to know i need a reboot?

is there a file that is flagged?
or does the upgrade process return something that i can catch using grep?
or better still is there a way to tell apt-get or aptitude to reboot the system
after an upgrade when necessary?

TIA,
Tomoki Taniguchi